It is all explained here : https://docs.microsoft.com/en-us/azure/active-directory/enterprise-users/groups-dynamic-membership
An example of active users is the below, taken from the link above:
Properties Allowed values Usage
accountEnabled true false user.accountEnabled -eq true
dirSyncEnabled true false user.dirSyncEnabled -eq true
Keep in mind that this feature requires an Azure AD Premium P1 license for each unique user that is a member of one or more dynamic groups. You don't have to assign licenses to users for them to be members of dynamic groups, but you must have the minimum number of licenses in the Azure AD organization to cover all such users. For example, if you had a total of 1,000 unique users in all dynamic groups in your organization, you would need at least 1,000 licenses for Azure AD Premium P1 to meet the license requirement.